Why keep nuclear weapons at all? Because, as we're often reminded, small countries with big ambitions and dodgy governments (Iran, North Korea etc. - Pakistan already has them and that place is hardly a model of democratic stability) are getting them and we need to wave an even bigger stick in their faces just to keep them quiet.
If they ever have more powerful weapons than we have, they won't be as gracious about it as we have been so far, that's for sure. The reason we're all still here is because of the good old policy of mutually assured destruction - If you show me yours, I'll definitely show you mine.
In today's environment it would make more sense to replace ballistic nuclear missiles launched from submarines with cruise missiles launched from anywhere we choose (including submarines) Cruise missiles are more flexible and could carry conventional or nuclear warheads as befits the situation and the target. I don't see the need for one huge bang when several smaller bangs bracketing the target can be far more effective. There are cases where you can't use a really, really big bang but a 'small' 5 or 10 kiloton device would do just fine.